1. Field of the Invention
This invention relates generally to the field of optically reflective filters and materials, and, more particularly, to a multiple-notch rugate filter manufactured under a monitored and feedback-controlled deposition process.
2. Description of Related Art
An important criterion in the operation of optical sensors is protecting the sensors from possible damage caused by lasers. This protection is referred to as laser hardening and has become necessary due to the widespread use of lasers in industrial and military applications, such as ranging and communications.
Such protection is needed because laser radiation from friendly or hostile sources, even at lower energy levels, can disable or damage a detection system by saturating or burning out vulnerable components or detector elements. While protecting such systems, low spectral distortion and high see-through are needed to depict an accurate signal.
In conventional laser-hardening schemes, a standard method of such protection is a multilayer dielectric reflective filter made of alternating layers of dissimilar materials. Among the problems associated with the use of such a discrete multilayer structure is a residual stress in each layer caused by incompatible material properties at the abrupt interfaces. This stress, a problem common to discrete multilayer coatings, can weaken the structure and lead to delamination of the layers. In addition, abrupt interfaces between dissimilar materials can be sites for dislocation and a high concentration of impurities. These sites can lower the protection capability and power tolerance of the filter if radiation is scattered into the detector or absorbed in the filter. Multilayer filters also have inadequate broadband signal transmittance due to the undesired sideband reflectance peaks that arise from the interference effects inherent to the filter design limitations of alternating layer structures.
Simple reflective multilayer dielectric filters typically consist of alternating layers of two dielectric materials of different refractive indices, which are formed on the surface of a substrate by known deposition techniques, such as chemical vapor deposition, sputtering, or thermal evaporation. The optical thickness (in this case, the product of the refractive index and layer thickness) of each layer is chosen to be one quarter of a wavelength of the radiation to be reflected, and such a structure is therefore referred to as a "quarter-wave stack". As previously noted, such multiple-layer filters exhibit numerous problems, including the production, upon intense irradiation, of highly localized fields occurring at the abrupt interfaces between the layer surfaces, which can produce temperature increases, which in turn can lead to structural failure due to interfacial stresses.
In order to improve upon optical materials for use in optical filters, U.S. Pat. No. 4,545,646 to Chern et al. (issued Oct. 8, 1985 and assigned to the present assignee) provided a graded-index optical material having continuous gradations in the stoichiometric composition and refractive index as a function of thickness of the material. The structure disclosed in U.S. Pat. No. 4,545,646 to Chern et al. addressed many of the drawbacks, such as design and performance limitations, in multilayer filters. Chern et al. discloses a method whereby the substrate is exposed to first and second vapor phase reactants in predetermined proportions in the presence of radiation to induce a chemical reaction whereby the desired optical material is formed and deposited on the substrate. The optical material is deposited in accordance with a predetermined index of refraction. The proportion of the reactants to which the substrate is exposed is altered as a function of time in a predetermined and continuous manner which allows the predetermined and continuous variation of the stoichiometric composition and the index of refraction of the deposited material to produce a graded index of refraction which reflects radiation at a given wavelength. In this single-notch process of Chern et al. it is assumed that the deposition process will occur according to the predetermined pattern. However, no method for insuring the accuracy of the deposition pattern was provided by Chern et al.
In order to provide protection against multiple laser lines, multiple single-notch rugate layers could be stacked one on the other. Each stack, however, requires a separate process segment with possible interfacial problems between stacks. Superposition of component sinusoidal modulations allows a single process segment for simultaneous multiple notch formations. Chern et al. also disclosed the formation of a multiple-notch rugate filter from a graded index coating having a composite index profile that is based on the linear superpositioning of a number of sinusoidal index profiles. For example, three separate index profiles can be combined to form a composite refractive index profile which provides protection against three separate wavelengths. The ability to control and insure the accuracy of the deposition pattern is especially crucial in preparing multiple-notch filters. However, rugate filter fabrication can tolerate errors no greater than about one percent to the original design guide, as disclosed by Southwell et al. in U.S. Pat. No. 4,707,611. Therefore, errors greater than one percent need to be compensated as they occur, or else the fabrication process requires termination and starting over.
Thus, a need exists in the field of optical filters for a method of forming multiple-notch rugate filters from graded index optical materials in a controllable and accurate manner.